ALSA supported PCM formats

ALSA supported PCM formats

[Assaf Hoffman]

HI,
I'm new to ALSA and I'm trying to understand the PCM formats and the way
ALSA stack place/pick them from the DMA buffer.

My understanding concerns PCM formats that their physical representation
bit size is bigger than the actual sound sample bit size.
For example, defined in pcm_misc.c file under pcm_formats[]:
	[SNDRV_PCM_FORMAT_x24_xE]  = {.width = 24, .phys = 32, ...}
	[SNDRV_PCM_FORMAT_x20_3xE] = {.width = 20, .phys = 24, ...}
	[SNDRV_PCM_FORMAT_x18_3xE] = {.width = 18, .phys = 24, ...}
How the ALSA stack (kernel 2.6.12.6) knows the offset of the sound
sample bits inside the format physical size? For example, when the sound
sample size is 24 (.width = 24) and the format physical size is 32
(.phys = 32) the sound sample offset inside the physical frame can
anywhere beteen 0-7 bit. How the ALSA stack can know where to look for
the sound sample?
Thanks. 

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Re: ALSA supported PCM formats

[Clemens Ladisch]

Assaf Hoffman wrote:
> I'm new to ALSA and I'm trying to understand the PCM formats and the way
> ALSA stack place/pick them from the DMA buffer.
> 
> My understanding concerns PCM formats that their physical representation
> bit size is bigger than the actual sound sample bit size.
> For example, defined in pcm_misc.c file under pcm_formats[]:
> 	[SNDRV_PCM_FORMAT_x24_xE]  = {.width = 24, .phys = 32, ...}
> 	[SNDRV_PCM_FORMAT_x20_3xE] = {.width = 20, .phys = 24, ...}
> 	[SNDRV_PCM_FORMAT_x18_3xE] = {.width = 18, .phys = 24, ...}
> How the ALSA stack (kernel 2.6.12.6) knows the offset of the sound
> sample bits inside the format physical size? For example, when the sound
> sample size is 24 (.width = 24) and the format physical size is 32
> (.phys = 32) the sound sample offset inside the physical frame can
> anywhere beteen 0-7 bit. How the ALSA stack can know where to look for
> the sound sample?

There are three sizes that describe a sample:
1) physical size: how many bits are occupied in memory, i.e., how many
   data is read from/written to the DMA buffer for each sample;
2) sample width: the number of bits, starting at the _least_ significant
   bit inside (1), used for sample data;
3) significant bits: the number of bits, starting at the _most_
   significant bit inside (2), actually used for sample data.

Which bit is least/most significant depends on whether the sample format
is little or big endian.


HTH
Clemens

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Re: ALSA supported PCM formats

[Assaf Hoffman]

Hi,
Thank you for your quick respond.
Regarding 2 and 3 below, aren't those different names for the same
thing?
If not, can you give me an example of a case where they are different?
Thanks.


-----Original Message-----
From: Clemens Ladisch [mailto:cladisch@fastmail.net] 
Sent: Thursday, October 19, 2006 11:23 AM
To: Assaf Hoffman; alsa-devel@lists.sourceforge.net
Cc: Rita Shtern
Subject: Re: [Alsa-devel] ALSA supported PCM formats
Importance: Low

Assaf Hoffman wrote:
> I'm new to ALSA and I'm trying to understand the PCM formats and the
way
> ALSA stack place/pick them from the DMA buffer.
> 
> My understanding concerns PCM formats that their physical
representation
> bit size is bigger than the actual sound sample bit size.
> For example, defined in pcm_misc.c file under pcm_formats[]:
> 	[SNDRV_PCM_FORMAT_x24_xE]  = {.width = 24, .phys = 32, ...}
> 	[SNDRV_PCM_FORMAT_x20_3xE] = {.width = 20, .phys = 24, ...}
> 	[SNDRV_PCM_FORMAT_x18_3xE] = {.width = 18, .phys = 24, ...}
> How the ALSA stack (kernel 2.6.12.6) knows the offset of the sound
> sample bits inside the format physical size? For example, when the
sound
> sample size is 24 (.width = 24) and the format physical size is 32
> (.phys = 32) the sound sample offset inside the physical frame can
> anywhere beteen 0-7 bit. How the ALSA stack can know where to look for
> the sound sample?

There are three sizes that describe a sample:
1) physical size: how many bits are occupied in memory, i.e., how many
   data is read from/written to the DMA buffer for each sample;
2) sample width: the number of bits, starting at the _least_ significant
   bit inside (1), used for sample data;
3) significant bits: the number of bits, starting at the _most_
   significant bit inside (2), actually used for sample data.

Which bit is least/most significant depends on whether the sample format
is little or big endian.


HTH
Clemens

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